A solid oxide fuel cell (referred to hereinafter merely as an SOFC where appropriate) is a fuel cell using an oxide as a solid electrolytic material having ionic conductivity. The fuel cell generally has an operating temperature as high as on the order of 1000° C., but there has lately been developed one having an operating temperature not higher than 800° C., for example, on the order of 750° C. With the SOFC, there are disposed a fuel electrode (that is, an anode), and an air electrode (that is, a cathode) with an electrolytic material sandwiched therebetween, thereby making up a single cell as a three-layer unit of the fuel electrode/an electrolyte/the air electrode. Although the air electrode is an oxygen electrode in the case of using oxygen as an oxidizing agent, it includes the oxygen electrode according to the invention.
When the SOFC is operated, fuel is fed to the fuel electrode side of the single cell (also referred to merely as “a cell” where appropriate in the present description), air and oxygen enriched air as an oxidizing agent or oxygen is fed to the air electrode side thereof, and electric power is obtained by connecting both the electrodes to an external load. However, with the single cell of one unit only, a voltage only on the order of 0.7V at most can be obtained, so that there is the need for connecting in series a plurality of the single cells together in order to obtain electric power for practical use. For the purpose of electrically connecting adjacent cells with each other while simultaneously feeding fuel, and air to the fuel electrode, and the air electrode, respectively, after properly distributing them, and subsequently, effecting emission thereof, separators (=interconnectors) and the single cells are alternately deposited.
Such a SOFC module is a type wherein a plurality of the single cells are stacked one on top of another, but it is conceivable to adopt a multi-segment type in place of such a type as described. For example, in Fifth European Solid Oxide Fuel Cell forum (1 to 5, July, 2002) p. 1075-, the external appearance, and so forth, of the multi-segment type are disclosed although the contents thereof are not necessarily clear-cut in detail. As the multi-segment type, two types including a cylindrical type, and a hollow flat type are conceivable.
FIGS. 1(a) to 1(c) are views showing an example of the structure of the hollow flat type of the two types, FIG. 1(a) being an oblique perspective view, FIG. 1(b) a plan view, and FIG. 1(c) a sectional view taken on line A-A in FIG. 1(b). As shown in FIGS. 1(a) to 1(c), there are formed a plurality of cells 2 each made up by stacking a fuel electrode 3, an electrolyte 4, and an air electrode 5 in that order on an insulator substrate 1 in a hollow flat sectional shape, and the respective cells 2 are structured so as to be electrically connected in series with each other through the intermediary of an interconnector 6, respectively. Fuel is caused to flow in space (=a hollow area) within the insulator substrate 1, that is, an internal fuel flow part 7, in parallel with a lineup of the cells 2, as indicated by an arrow (→) in FIGS. 1(a), and 1(c). In FIG. 1(c), the interconnector 6 is seen covering part of the surface of the air electrode 5, however, may cover the entire surface thereof. In this respect, the same can be said hereinafter.
For a constituent material of the insulator substrate in the hollow flat sectional shape, use can be made of a porous material capable of withstanding the operating temperature of an SOFC module, but use is normally made of a ceramic. For use in the electrolyte, a solid electrolytic material having ionic conductivity is sufficient, and use can be made of a sheet like a sintered body such as, for example, yttria-stabilized zirconia (YSZ). For the fuel electrode, use is made of a porous material such as, for example, a mixture of Ni and yttria-stabilized zirconia YSZ (Ni/YSZ cermet), and so forth. For the air electrode, use is made of a porous material, for example, Sr-doped LaMnO3, and so forth.
In fabrication of the respective cells, the fuel electrode, the electrolyte, and the air electrode are normally fabricated by separate processes by screen printing, and so forth, and those electrodes are deposited in that order on top of the insulator substrate in the hollow flat sectional shape to be subsequently sintered, thereby forming the respective cells. The respective adjacent cells are structured so as to be electrically connected in series with each other through the intermediary of the interconnector.